Optical filter and optical module provided with same

ABSTRACT

An optical filter includes first and second substrates, first and second mirrors, and first and second electrodes. The first substrate has a flat surface. The second substrate includes a first surface, a second surface and a third surface, the second surface surrounding the first surface in a plan view, the third surface surrounding the second surface in a plan view, a height of the first surface above the second surface being lower than a height of the third surface above the second surface, the first surface and the second surface facing the flat surface of the first substrate. The first mirror is disposed on the flat surface of the first substrate. The second mirror is disposed on the first surface of the second substrate, the second mirror facing the first mirror. The first electrode is disposed on the flat surface of the first substrate. The second electrode is disposed on the second surface of the second substrate, the second electrode facing the first electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/086,292 filed on Nov. 21, 2013, which is acontinuation application of U.S. patent application Ser. No. 13/655,963filed on Oct. 19, 2012, which is a continuation application of U.S.patent application Ser. No. 12/710,426 filed on Feb. 23, 2010, now U.S.Pat. No. 8,319,169. This application claims priority to Japanese PatentApplication No. 2009-050791 filed on Mar. 4, 2009. The entiredisclosures of U.S. patent application Ser. Nos. 14/086,292, 13/655,963and 12/710,426 and Japanese Patent Application No. 2009-050791 arehereby incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to an optical filter and to an opticalmodule that is provided with the optical filter.

2. Related Art

Conventional air-gap-type electrostatically actuated optical filters areknown as optical filters for selectively emitting a desired wavelengthof light from among all the wavelengths of an incident light. In suchfilters, a pair of substrates are arranged facing each other, a mirroris provided to each of the opposing surfaces of the substrates,electrodes are provided on the periphery of the mirrors, a diaphragmportion is provided on the periphery of one mirror, and the diaphragmportion is displaced by electrostatic force between the electrodes tovary the gap (air gap) between the mirrors, whereby the desiredwavelength of light is extracted (see Japanese Laid-Open PatentPublication No. 2003-57438 and Japanese Laid-Open Patent Publication No.2008-116669, for example).

In this type of optical filter, the wavelength of light that correspondsto the gap between the mirrors can be selectively extracted by varyingthe gap between the mirrors.

SUMMARY

An optical filter according to one aspect includes first and secondsubstrates, first and second mirrors, and first and second electrodes.The first substrate has a flat surface. The second substrate includes afirst surface, a second surface and a third surface, the second surfacesurrounding the first surface in a plan view, the third surfacesurrounding the second surface in a plan view, a height of the firstsurface above the second surface being lower than a height of the thirdsurface above the second surface, the first surface and the secondsurface facing the flat surface of the first substrate. The first mirroris disposed on the flat surface of the first substrate. The secondmirror is disposed on the first surface of the second substrate, thesecond mirror facing the first mirror. The first electrode is disposedon the flat surface of the first substrate. The second electrode isdisposed on the second surface of the second substrate, the secondelectrode facing the first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is a schematic plan view showing the optical filter according toan embodiment of the present invention;

FIG. 2 is a schematic sectional view showing the optical filteraccording to an embodiment of the present invention;

FIG. 3 is a view showing the relationship between the wavelength andtransmittance in the optical filter according to an embodiment of thepresent invention;

FIG. 4 is a schematic sectional view showing the optical sensoraccording to an embodiment of the present invention; and

FIG. 5 is a schematic structural view showing an embodiment of theoptical module of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Preferred embodiments of the optical filter and optical module providedwith the optical filter of the present invention will next be described.

An air-gap-type electrostatically actuated optical filter will bedescribed as the optical filter.

In the following description, an XYZ orthogonal coordinate system isset, and the positional relationships of members will be described withreference to this XYZ orthogonal coordinate system as needed. In thissystem, a predetermined direction in the horizontal plane is designatedas the X-axis direction, the direction orthogonal to the X-axisdirection in the horizontal plane is designated as the Y-axis direction,and the direction orthogonal to the X-axis direction and Y-axisdirection (i.e., the vertical direction) is designated as the Z-axisdirection.

FIG. 1 is a plan view showing the optical filter of the presentembodiment, and FIG. 2 is a sectional view showing the optical filter ofthe present embodiment. In FIGS. 1 and 2, the reference numeral 1 refersto an optical filter composed of an air-gap-type electrostaticallyactuated etalon element.

The optical filter 1 of the present embodiment is composed of a firstsubstrate 2; a second substrate 3 joined (or bonded) to the firstsubstrate 2 so as to face the first substrate 2; a circular first mirrorpair 4 provided at the center of the surfaces 2 a, 3 a on the opposingsides of the first substrate 2 and the second substrate 3, respectively;a ring-shaped second mirror pair 5 provided on the periphery of thefirst mirror pair 4; a ring-shaped electrode pair 6 provided on theperiphery of the second mirror pair 5; and a diaphragm portion 7.

The first mirror pair 4 and second mirror pair 5 are each composed of apair of mirrors facing each other, and among the first mirror pair 4 andthe second mirror pair 5, the pair of mirrors provided to the deformablefirst substrate 2 are referred to as movable mirrors 4A, 5A (a mirror 4Ais an example of a first mirror), and the pair of mirrors provided tothe non-deforming second substrate 3 are referred to as fixed mirrors4B, 5B (a mirror 4B is an example of a second mirror).

A step 21 is formed at the peripheral edge of an opposing surface 2 a ofthe first substrate 2, a movable portion 22 composed of a concaveportion is formed by the step 21, and a first movable mirror 4A, asecond movable mirror 5A, and an electrode (a first electrode) 6A areaccommodated within the movable portion 22.

A first step (a first surface) 31 that forms a circle is formed in thecenter of an opposing surface 3 a of the second substrate 3, and a firststep portion 32 (light transmissive portion) composed of the concaveportion formed by the first step 31 is formed in the center of theopposing surface 3 a of the second substrate 3. A ring-shaped secondstep (a second surface) 33 concentric with the first step portion 32 isformed on the peripheral edge of the opposing surface 3 a of the secondsubstrate 3, and a second step portion 34 composed of the concaveportion formed by the second step 33 is formed on the peripheral edge ofthe opposing surface 3 a of the second substrate 3. A third surface 37surrounds the second step 33 in the plan view. As shown in FIG. 2, aheight of the second surface in the second step 33 is lower than aheight of the first surface in the first step 31, and a height of thethird surface 37 is higher than the height of the first surface in thefirst step 31.

A third step 35 composed of a naturally ring-shaped convex portion isformed in the portion flanked by the two concave portions describedabove provided to the opposing surface 3 a, i.e., at the boundarybetween the first step portion 32 and the second step portion 34, and athird step portion 36 (light transmissive portion) is provided.

The top surface and the bottom surfaces of the movable step portion 22,first step portion 32, second step portion 34, and third step portion 36are parallel to each other; and the first mirror pair 4, second mirrorpair 5, and electrode pair 6 provided to the step portions are eachmaintained parallel to each other via a respective gap.

A first fixed mirror 4B is provided via a first gap G1 to the bottomportion of the first step portion 32 so as to face and form a pair witha first movable mirror 4A. In the same manner, a second fixed mirror 5Bis provided via a second gap G2 to the top surface of the third stepportion 36 so as to face and form a pair with a second movable mirror5A. A fixed electrode (a second electrode) 6B is also provided via athird gap G3 to the bottom portion of the second step portion 34 so asto face and form a pair with a movable electrode 6A.

A ring-shaped diaphragm portion 7 having a small wall thickness formedby etching (selective removal) is formed on an external side surface 2 bof the first substrate in a position that substantially corresponds tothe external peripheral portion of the movable electrode 6A, and thediaphragm portion 7 and the electrode pair 6 provided facing each othervia the third gap G3 constitute an electrostatic actuator.

The first substrate 2 and second substrate 3 are both rectangles(squares) of optically transparent material (light transmissivematerial) having insulation properties, and are preferably composedparticularly of glass or another transparent material.

Specific examples of glass that can be suitably used include soda glass,crystallized glass, quartz glass, lead glass, potassium glass,borosilicate glass, sodium borosilicate glass, non-alkali glass, and thelike.

Making both the first substrate 2 and the second substrate 3 anoptically transparent material enables electromagnetic waves or visiblelight rays having the desired wavelength spectrum to be used as theincident light. Moreover, forming both the first substrate 2 and thesecond substrate 3 out of a semiconductor material, e.g., silicon,enables near-infrared rays to be used as the incident light.

The first movable mirror 4A, the first fixed mirror 4B, the secondmovable mirror 5A, and the second fixed mirror 5B are composed ofdielectric multilayer films in which a plurality ofhigh-refractive-index layers and low-refractive-index layers is layeredin alternating fashion, and the mirrors may have the same composition aseach other or different compositions. The first mirror pair 4 and secondmirror pair 5 are not limited to dielectric multilayer films, and acarbon-containing silver alloy film or the like, for example, may alsobe used.

When the optical filter 1 is used in the visible light region or theinfrared region, titanium oxide (Ti₂O), tantalum oxide (Ta₂O₅), niobiumoxide (Nb₂O₅), or the like, for example, is used as the material forforming the high-refractive-index layers in the dielectric multilayerfilm. When the optical filter 1 is used in the ultraviolet region,aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂),thorium oxide (ThO₂), or the like, for example, is used as the materialfor forming the high-refractive-index layers. Magnesium fluoride (MgF₂),silicon dioxide (SiO₂), or the like, for example, is used as thematerial for forming the low-refractive-index layers in the dielectricmultilayer film.

The thickness and number of layers of high-refractive-index layers andlow-refractive-index layers are appropriately set based on the requiredoptical characteristics. In general, when a reflective film (mirror) isformed by a dielectric multilayer film, the number of layers needed toobtain the optical characteristics is 12 or more.

In the electrode pair 6, an electrostatic force is generated between theelectrodes 6A, 6B according to an inputted drive voltage, and theelectrode pair 6 constitutes an electrostatic actuator for moving thefirst mirror pair 4 and second mirror pair 5 relative to each other in astate in which the mirrors are facing each other.

The electrode pair 6 is configured so that the diaphragm portion 7 isdisplaced in the vertical direction in FIG. 2, the first gap G1 betweenthe first mirror pair 4 and the second gap G2 between the second mirrorpair 5 are each varied, and wavelengths of light corresponding to thefirst gap G1 and the second gap G2 are individually emitted.

The material for forming the electrode pair 6 is not particularlylimited insofar as the material is conductive, and examples of materialsthat can be used include Cr, Al, Al alloy, Ni, Zn, Ti, Au, and othermetals; resins in which carbon, titanium, or the like is dispersed;polycrystalline silicon (polysilicon), amorphous silicon, and othersilicon; silicon nitride, ITO, and other transparent conductivematerials; and other materials.

As shown in FIG. 1, interconnections 11A, 11B are connected to theelectrodes 6A, 6B, and the electrodes 6A, 6B are connected to a powersupply (not shown) via the interconnections 11A, 11B.

The interconnections 11A, 11B are formed in an interconnection groove12A formed in the first substrate 2, or an interconnection groove 12Bformed in the second substrate 3. Consequently, there is no interferenceat the junction of the first substrate 2 and second substrate 3.

A power supply drives the electrodes 6A, 6B by application of a voltageto the electrodes 6A, 6B as a drive signal, and generates a desiredelectrostatic force between the electrodes 6A, 6B. A control device (notshown) is connected to the power supply, and the power supply iscontrolled by the control device, whereby the difference of potentialbetween the electrodes 6A, 6B can be adjusted.

The diaphragm portion 7 is thinner than the portion of the firstsubstrate 2 in which the diaphragm portion 7 is not formed. The area ofthe first substrate 2 thinner than the remainder thereof is thereforeelastic (flexible) and capable of deformation (displacement). Throughthis configuration, the diaphragm portion 7 varies the first gap G1 andsecond gap G2 to change the gap intervals of the first mirror pair 4 andsecond mirror pair 5 to the interval that corresponds to the desiredwavelength of light. The optical filter 1 thus has a wavelengthselection capability for emitting desired wavelengths of light.

The shape or thickness of each of the diaphragm portions 7 is arbitraryinsofar as light in the desired wavelength range is emitted.Specifically, these characteristics are set with consideration for theamount of variation, rate of variation, and other characteristics of theintervals of the gaps G1, G2 of the first mirror pair 4 and secondmirror pair 5, and in accordance with the wavelength range of emittedlight needed from the optical filter 1.

A circular step 21 is formed in the first substrate 2 to provide themovable step portion 22 in the present embodiment, but a plurality ofmovable step portions divided for each mirror and electrode may also beformed on the opposing surface 2 a in the same manner as in the secondsubstrate 3. Conversely, a configuration may also be adopted in which nosteps are provided to the first substrate 2, and the opposing surface 2a is directly utilized as a movable portion.

The shape and height of each step of the second substrate 3 are also notlimited by the present embodiment, and it is sufficient insofar as thegaps G1, G2 between the mirror pairs are each different. For example,the first step portion 32 may be a convex portion and the third stepportion 36 may be a concave portion.

In the present embodiment, the first fixed mirror 4B, the second fixedmirror 5B, and the fixed electrode 6B are provided to the opposingsurface 3 a each via a gap G1, G2, G3 of a different size, but aconfiguration may also be adopted in which any one of the first fixedmirror 4B and second fixed mirror 5B is positioned in the same plane asthe fixed electrode 6B, and the size of one of the first gap G1 andsecond gap G2 is equal to that of the third gap G3.

Moreover, the number of mirror pairs provided to the optical filter 1 isnot limited by the present embodiment, and more mirrors may be provided,but an excessively large number of mirrors is impractical. This is dueto the increased cost of forming the same number of steps as mirrors onthe substrate surface, and the space requirements for aligning numerousmirrors.

Light division using the optical filter 1 of the present embodiment willnext be described.

In the optical filter 1 of the present embodiment, when a voltage is notapplied between the electrode 6A and electrode 6B, first movable mirror4A and the first fixed mirror 4B face each other across the first gapG1. In the same manner, the second movable mirror 5A and the secondfixed mirror 5B face each other across the second gap G2.

Therefore, when light is incident on the optical filter 1, a wavelengthof light that corresponds to the first gap G1 is emitted by the firstmirror pair 4; e.g., light having a wavelength of 480 nm is emitted, asindicated by the solid line in FIG. 3. A wavelength of light thatcorresponds to the second gap G2, e.g., light having a wavelength of 630nm, is emitted by the second mirror pair 5, as indicated by the dashedline in FIG. 3.

Since the first mirror pair 4 and the second mirror pair 5 are eacharranged parallel in an independent state, light that corresponds to thegap G1, G2 of each mirror pair is diffracted simultaneously withoutinterference. In the optical filter 1, two different wavelengths oflight can thus be diffracted simultaneously without interfering witheach other.

When the control device and power supply are driven, and a voltage isapplied between the electrode 6A and the electrode 6B, an electrostaticforce corresponding to the size of the voltage (potential difference) isgenerated between the electrode 6A and electrode 6B. The control devicethus controls the power supply, whereby the desired voltage can beapplied between the electrodes 6A, 6B, and the desired electrostaticforce can be generated between the electrode 6A and electrode 6B.

When the desired electrostatic force is generated between the electrodes6A, 6B, the electrodes 6A, 6B are pulled toward each other by theelectrostatic force, and the first substrate 2 deforms so as to flextoward the second substrate 3. The first gap G1 of the first mirror pair4 and the second gap G2 of the second mirror pair 5 are then maintainedas gaps G1′, G2′ that are smaller than when a voltage was not applied.

When light is incident on the optical filter 1 in this state, thetransmitted wavelengths are both shifted to shorter wavelengths thatcorrespond to the first gap G1′ and the second gap G2′.

Light having a wavelength of 400 nm, for example, is emitted by thefirst mirror pair 4, as indicated by the single-dot dashed line in FIG.3, and light having a wavelength of 550 nm, for example, is emitted bythe second mirror pair 5, as indicated by the double-dot dashed line inFIG. 3.

Two different wavelengths can thus be simultaneously shifted in theoptical filter 1 and individually emitted.

Through the optical filter 1 of the present embodiment as describedabove, by providing a plurality of step portions 32, 35 composed ofsteps 31, 33 having mutually different heights to the opposing surface 3a of the second substrate 3, gaps G1, G2 having different sizes areformed between the first substrate 2 and the second substrate 3, and byproviding two mirror pairs 4, 5 to the step portions via the gaps G1,G2, an optical filter can be provided that is capable of diffractinglight into a plurality of wavelengths individually and simultaneously.

An embodiment of an optical sensor provided with the optical filter 1will next be described as an example of an application of the opticalfilter 1 of the present embodiment.

FIG. 4 is a schematic sectional view showing the structure of theoptical sensor of the present embodiment, wherein the reference numeral10 refers to the optical sensor of the present embodiment. The samereference numerals are used in FIG. 4 to refer to members that are thesame as those of the optical filter 1 shown in FIGS. 1 and 2, and nofurther description of such members will be given.

The optical sensor 10 is provided with a photoreceptor 9 on the emissionside of the optical filter 1, and a plurality of photoreceptor elements94, 95 is provided for individually detecting light that is diffractedby the mirror pairs 4, 5 of the optical filter 1 and emitted. Thephotoreceptor elements 94, 95 are composed of photodiodes or the like,for example, and are arranged so as to face the emission surfaces of themirror pairs 4, 5, respectively. The photoreceptor elements 94, 95receive the emitted light from the optical filter 1 and convert thereceived light to an electrical signal.

By thus providing a plurality of photoreceptor elements so as tocorrespond to the number of provided mirror pairs in the optical filter1, the different wavelengths of light emitted simultaneously andindividually by the optical filter 1 can be independently detected.

FIG. 5 is a view showing an embodiment of the optical filter devicemodule of the present invention provided with the optical sensor 10, andin FIG. 5, the reference numeral 50 refers to an optical filter devicemodule.

The optical filter device module 50 is provided with a filter unit 51composed of the optical filter 1 of the present embodiment, and adetection element 55 composed of the photoreceptor 9 of the presentembodiment, and the optical filter device module 50 is configured sothat a specific spectrum of light is radiated to a specimen W, a pre-setwavelength of light is selectively extracted (diffracted) from the lightreflected by the specimen W, and the intensity of the extracted light ismeasured.

Specifically, the optical filter device module 50 is provided with alight source optical system 54 for radiating a predetermined light,e.g., visible light or infrared rays, to the specimen W, the lightsource optical system 54 having a light source 52 and a lens 53; adetector optical system 56 for detecting reflected light from thespecimen W, the detector optical system 56 having a filter unit 51 and adetection element 55; a light source control circuit 57 for controllingthe illumination intensity and other characteristics of the light source52; a filter control circuit 58 for controlling the filter unit 51; anda processor 59 for receiving detection signals detected by the detectionelement 55, the processor 59 being connected to the light source controlcircuit 57 and the filter control circuit 58.

In such an optical filter device module 50, a specific spectrum of lightsuch as visible light or infrared rays is radiated to the specimen W.Light is then reflected according to the surface state of the specimenW, for example, and other factors, and the reflected light enters thefilter unit 51. The filter unit 51 is configured so that a voltage isapplied (or not applied) to the electrodes 6A, 6B so that light having apre-set wavelength is selectively extracted (diffracted). Only aspecific wavelength band is thereby selectively extracted from thereflected light and detected by the detection element 55. Consequently,reflected light can be detected with high sensitivity by using adetection element that selectively detects the light extracted by thefilter unit 51 as the detection element 55, for example. The opticalfilter device module 50 thereby enables the surface state and othercharacteristics of the specimen W to be detected with high sensitivity.

In such an optical filter device module 50, since the optical filter 1is provided in the filter unit 51, the spectral characteristics of theoptical filter 1 can be utilized without modification.

For example, since the optical filter 1 as previously described has suchspectral characteristics as shown in FIG. 3, a case will be described inwhich reflected light is detected when light in the wavelength region of400 to 700 nm is sequentially scanned and radiated to a specimen W.

Through the shift of transmitted wavelengths such as shown in FIG. 3,light already in the wavelengths of 400 to 550 nm can be diffracted bythe first mirror pair 4, and light already in the wavelengths of 550 to700 nm can be diffracted by the second mirror pair 5. Since thisdiffraction can be independently and simultaneously performed by eachmirror pair, the entire wavelength region to be measured can be dividedinto a 400 to 550 nm short-wavelength region and a 550 to 700 nmlong-wavelength region.

The short-wavelength region is thus sequentially scanned by the firstmirror pair 4 and the photoreceptor element 94 and detected, while atthe same time, the long-wavelength region is sequentially scanned by thesecond mirror pair 5 and the photoreceptor element 95 and detected. Thewavelength region to be scanned can thereby be reduced substantially byhalf in comparison to sequentially scanning and detecting the entirewavelength region, and the time needed for detection and measurement canbe significantly reduced.

Moreover, even though the optical filter 1 of the present embodiment hasmirror pairs 4, 5 that have different spectral characteristics, theoptical filter 1 is still substantially the same size as theconventional optical filter. Less space is therefore required relativeto a case in which conventional optical filters are arranged within amodule, and a smaller-sized optical filter device module 50 can beprovided.

General Interpretation of Terms

In understanding the scope of the present invention, the term“comprising” and its derivatives, as used herein, are intended to beopen ended terms that specify the presence of the stated features,elements, components, groups, integers, and/or steps, but do not excludethe presence of other unstated features, elements, components, groups,integers and/or steps. The foregoing also applies to words havingsimilar meanings such as the terms, “including”, “having” and theirderivatives. Also, the terms “part,” “section,” “portion,” “member” or“element” when used in the singular can have the dual meaning of asingle part or a plurality of parts. Finally, terms of degree such as“substantially”, “about” and “approximately” as used herein mean areasonable amount of deviation of the modified term such that the endresult is not significantly changed. For example, these terms can beconstrued as including a deviation of at least ±5% of the modified termif this deviation would not negate the meaning of the word it modifies.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. Furthermore, the foregoing descriptions of theembodiments according to the present invention are provided forillustration only, and not for the purpose of limiting the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. An optical filter comprising: a first substratehaving a flat surface; a second substrate including a first surface, asecond surface and a third surface, the second surface surrounding thefirst surface in a plan view, the third surface surrounding the secondsurface in a plan view, a height of the first surface above the secondsurface being lower than a height of the third surface above the secondsurface, the first surface and the second surface facing the flatsurface of the first substrate; a first mirror disposed on the flatsurface of the first substrate; a second mirror disposed on the firstsurface of the second substrate, the second mirror facing the firstmirror; a first electrode disposed on the flat surface of the firstsubstrate; and a second electrode disposed on the second surface of thesecond substrate, the second electrode facing the first electrode. 2.The optical filter according to claim 1, wherein the first substrateincludes a first portion and a second portion surrounding the firstportion in the plan view, a first thickness of the first portion isthicker than a second thickness of the second portion; and the firstmirror is disposed on the flat surface in the first portion of the firstsubstrate.
 3. The optical filter according to claim 1, wherein the firstsubstrate includes a first portion and a groove portion surrounding thefirst portion in the plan view as viewed from an opposite side to theflat surface, and the first mirror is disposed on the flat surface inthe first portion of the first substrate.
 4. The optical filteraccording to claim 1, wherein the first substrate and the secondsubstrate are made of light transmissive material.
 5. The optical filteraccording to claim 1, wherein at least one of the first substrate andthe second substrate is made of glass.
 6. The optical filter accordingto claim 1, wherein a surface area of the flat surface of the firstsubstrate is larger than a surface area of the first surface of thesecond substrate.
 7. An optical module comprising the optical filteraccording to claim
 1. 8. An optical filter comprising: a first substratehaving a surface; a second substrate having a first surface, a secondsurface, and a third surface, the second surface surrounding the firstsurface, the third surface surrounding the second surface, the firstsurface and the second surface facing the surface; a first mirrordisposed on the surface; a second mirror disposed on the first surface,the second mirror facing the first mirror; a first electrode disposed onthe surface; and a second electrode disposed on the second surface, thesecond electrode facing the first electrode, a first distance betweenthe surface and the first surface being smaller than a second distancebetween the surface and the second surface, a third distance between thefirst surface and the second surface being smaller than a fourthdistance between the second surface and the third surface.
 9. An opticalmodule comprising the optical filter according to claim 8.